1. Technical Field
This invention relates to threaded fasteners for attaching the main rotor head to the main rotor mast of a helicopter, and more particularly to a nut retainer for ensuring that the rotor head assembly and rotor mast do not disengage even if the main rotor mast nut cracks or otherwise fails.
2. Background Information
Helicopters today typically have a main rotor head to which the rotor blades are attached. This main rotor head is in turn fastened to a main rotor mast by means of a main rotor mast nut (commonly called a "Jesus" nut). Viewing FIG. 1, which depicts a typical prior art attachment scheme for a main rotor head and mast assembly 8, a main rotor head 10 is attached to a main rotor mast 16 by means of a main rotor mast nut 20. Internal threads 22 on the nut 20 engage with external threads 34 on the mast 16 when the nut 20 is tightened down onto the mast 16, thereby completing the attachment of the assembly 10 to the mast 16. A plurality of threaded jack bolts 36 extend through the substantially round nut 20 and are torqued sequentially to engage respective threaded apertures (not shown) in a bearing inner race 38, thereby tightening down the nut 20 and properly pre-tensioning the mast 16, while requiring several orders of magnitude less torque than would be required by the nut 20 if it were tightened directly. The main rotor head 10 includes a plurality of rotor thrust bearings 43, which each comprise the bearing inner race 38 referenced above and a roller 44, the rollers 44 being protected from dirt and other contaminants by a seal 46. Seal 46 is retained in position outboard of the nut 20 by means of a seal retainer 48, which comprises a cup-shaped round annular disk which circumscribes the main mast 16 outwardly of the nut 20 and has an upwardly extending lip 50. A substantial gap or space 52 exists between the outer circumferential edge of the nut 20 and the inner circumferential edge of the retainer 48.
It can sometimes occur, during the course of repeated operation of the helicopter and its associated rotor system, that the nut 20 develops cracks or other such structural weaknesses due to fatigue and vibrational stresses which may over time cause the nut to fail during flight. Such a failure would permit the main rotor head 10, including the rotor blades (not shown) to separate from the main rotor mast 16, causing an inevitable crash with probable catastrophic consequences.
One present solution to the problem of main rotor nut cracking and failure is to establish a visual inspection program. Such a program, although labor intensive and expensive in terms of aircraft down-time, is essential for so critical a part. However, it is not foolproof, since cracks could propagate between inspections and cause a failure before being detected by inspection personnel. Also, it is possible that less obvious cracks could be missed by the inspectors. Unless inspections are conducted after every flight, a monumentally expensive and impractical undertaking, it is clear that a further solution is needed to supplement a periodic inspection program.
A second solution to the cracking problem is to manufacture the nut from a more crack-resistant material. This will reduce the risk that cracks will propagate in the nut, and thus reduce the risk of nut failure. However, this solution is also not a complete one, since no material is totally immune to failure.
What is needed is a means for retaining the nut in position on the mast even if it fails, essentially serving as a back-up safety device which would allow a pilot to guide the helicopter to a safe emergency landing and thereby avoid a catastrophic result.